There is known a hybrid vehicle including an internal combustion engine, a drive motor, and a power storage device for supplying power to the drive motor. In such a vehicle, it is necessary to diagnose, with high accuracy, whether the power storage device has deteriorated or not.
Japanese Patent Laying-Open No. 2000-131404 discloses a deterioration degree determining apparatus for diagnosing whether a power storage device has deteriorated or not, based on an amount of discharge from a fully-charged state to a prescribed discharge voltage value.